Digital Noise Reduction and Just Noticeable Differences: 2015

Digital Noise Reduction and Just Noticeable Differences: 2015

Pittman (2011) reported modern digital noise reduction (DNR) circuits do not have a negative impact on speech perception, and for older children, DNR significantly improved word learning rates in noise. Dillon (2012) reported that directional microphones and modern DNR circuits are beneficial and should be used all the time for children and adults. Pittman (2013) reported that modern DNR circuits can improve the signal-to-noise ratio (SNR) up to 6 dB. Ng and colleagues (2013) reported that DNR circuits “virtually cancelled out” the negative effects of background noise with regard to recall and DNR allowed more rapid word identification.

Magnusson et al (2013) reported that while using closed ear molds with DNR and directional microphones together, a statistically significant improvement in SNR occurred. Lowery and Plyler (2013) report that DNR and directional microphones are used to reduce the deleterious effects of background noise with regard to modern hearing aid amplification systems. Desjardins and Doherty (2014) evaluated listening effort with and without DNR across 12 adult experienced hearing aid wearers and stated that, “The NR algorithm used in this study significantly reduced” listening effort in the more difficult listening situation and they concluded NR significantly reduces listening effort for older patients.

Nonetheless, the just noticeable difference (JND) for SNR for speech has rarely been reported. McShefferty et al (2015) reported that the only previous published article on JND for SNR was by Killion (2004), which indicated subjects performed at chance level given a 2 dB SNR difference and achieved 90 percent correct given a 4 dB SNR. Killion reported that a 2 dB SNR advantage was likely beneficial, however, the benefit was unlikely to be noticed in a real-world environment.

McShefferty et al evaluated 69 people with multiple levels of hearing loss and normal hearing and reported on average a JND of SNR occurred at 3 dB. They suggest DNR in hearing aids “may need to achieve a benefit greater than 3 dB to be reliably discriminable….” That is, despite multiple advantages of DNR (see paragraph one, above) the SNR improvement generally needs to be better than 3 dB for the patient to perceive an improvement in the SNR.